Many types of surfactants have been proposed in the prior art for enhanced oil recovery. For reasons of cost and availability, the most used surfactants are of the sulfonate type, more commonly particularly petroleum sulfonates, available as alkali metals or ammonium salts. The use of these surfactants is satisfactory as long as the salt content of water does not exceed about 30 g/l (of sodium chloride equivalent), this value being given as an order of magnitude: particularly, the interfacial tensions between oil and sulfonate solutions, obtained by a judicious selection of the product characteristics, are vey low, of the order of 10.sup.-3 mN/m, or even less. But, when the salt content substantially exceeds the above-mentioned value, it has been observed that the interfacial properties of the sulfonates are quickly reduced as the content of divalent cations, calcium and magnesium in particular, is higher. Moreover, the high sensitivity of the sulfonates to divalent cations produces, during the progress of the the surfactants solution through the reservoir, precipitation phenomena and/or transfer of surfactant into a stationary phase, which phenomena, in relation with cation release from the rock, tends to make the surfactant inoperative.
It has been proposed to replace petroleum sulfonates with other anionic surfactants: for example paraffin-sulfonates, olefin-sulfonates, alkyl-sulfates, alkyl-phosphates, alkanoates, ethoxylated fatty alcohol and ethoxylated alkylphenol carboxylates, sulfates and sulfonates, etc. as well as non-ionic surfactants: for example ethoxylated fatty alcohols, ethoxylated alkylphenols, etc.
However, these surfactant substitutes for petroleum sulfonates are subject to a substantial loss of interfacial efficiency when the salt content of the field waters is high. The non-ionic surfactants are much less sensitive than the anionic surfactants to the presence of divalent cations, as far as the precipitation liability is concerned, however, they have the major disadvantage that their properties, when in solution, are very sensitive to small temperature variations. Moreover, the distribution of this type of products (distribution in relation with the polydispersity) between the various liquid phases is such that it results in a decrease of its useful concentration in the solution.
The various disadvantages from which the above-mentioned usual surfactants suffer have led to a search for other surface-active compounds whose surface activity is not substantially affected by the presence of polyvalent cations within large pH and temperature ranges.
It has thus been recommended to use N-acyl .alpha.-amino-carboxylic compounds as surfactants in the preparation of micellar systems for enhanced oil recovery. Thus, the French Pat. No. 2 440 773 and the first patent of addition thereto No. 2 468 402 describe the use of compounds complying with the general formula ##STR2## wherein R and R' are aliphatic radicals, R containing from 6 to 32 carbon atoms or more (e.g. up to 120) and R'1 to 18 carbon atoms. It is indicated that these compounds are used as salts formed with alkali metals (also optionally with alkaline-earth metals) and with primary, secondary or tertiary amines. Specific compounds used in prior documents are: sodium N-acetyl .alpha.-amino-tetradecanoate, sodium N-acetyl .alpha.-amino hexadecanoate, sodium N-acetyl .alpha.-amino octadecanoate, sodium N-acetyl .alpha.-amino tetracosanoate.
The corresponding salts of potassium, ammonium or of propylamine are also mentioned, as well as:
sodium N-propionyl .alpha.-amino hexadecanoate PA0 magnesium N-butyryl .alpha.-amino hexadecanoate PA0 ammonium N-propionyl .alpha.-amino eicosanoate PA0 potassium N-butyryl .alpha.-amino triacontanoate and the compound of formula: ##STR3## wherein R is a propylene oligomer containing about 20 units of CH.sub.3 --CH.dbd.CH.sub.2. PA0 sodium N-acetyl N-methyl .alpha.-amino octanoate PA0 ammonium N-acetyl N-propyl .alpha.-amino octanoate PA0 sodium N-lauryl N-methyl .alpha.-amino dodecanoate (with lauryl=C.sub.11 H.sub.23 --CO--) PA0 sodium N-acetyl N-methyl .alpha.-amino tetradecanoate PA0 sodium N-butyryl N-methyl .alpha.-amino eicosanoate, and sodium PA0 N-acetyl N-methyl .alpha.-amino docosanoate, tetracosanoate and hexacosanoate, used separately or as mixtures.